A liquid crystal display comprises a liquid crystal cell and a polarizing plate. In a liquid crystal display of transmission type, two polarizing plates are placed on both sides of the liquid crystal cell. On the other hand, a liquid crystal display of reflection type comprises a reflection plate, a liquid crystal cell and one polarizing plate in this order.
The liquid crystal cell comprises a pair of substrates, rod-like liquid crystal molecules and an electrode layer. The rod-like liquid crystal molecules are provided between the substrates. The electrode layer has a function of applying a voltage to the rod-like liquid crystal molecules. Various display modes are proposed according to alignment of the rod-like liquid crystal molecules in the cell. Examples of the display modes for transmission type include TN (twisted nematic) mode, IPS (in-plane switching) mode, FLC (ferroelectric liquid crystal) mode, OCB (optically compensatory bend) mode, STN (super twisted nematic) mode, ECB (electrically controlled birefringence) mode and VA (vertically aligned) mode. Examples of the modes for reflection type include TN mode and HAN (hybrid aligned nematic) mode.
A liquid crystal display usually comprises an optical compensatory sheet (phase retarder) as well as the liquid crystal cell and the polarizing plate. The optical compensatory sheet prevents the displayed image from undesirable coloring. The optical compensatory sheet has another function of enlarging a viewing angle of a liquid crystal cell. In a display of transmission type, one or two optical compensatory sheets are placed between the liquid crystal cell and the polarizing plate. In a display of reflection type, one optical compensatory sheet is placed between the liquid crystal cell and the polarizing plate.
A stretched birefringent polymer film has been conventionally used as the optical compensatory sheet.
An optical compensatory sheet comprising a transparent support and an optically anisotropic layer formed from liquid crystal molecules (particularly, discotic liquid crystal molecules) has recently been proposed in place of the stretched birefringent polymer film. The optically anisotropic layer is formed by aligning the liquid crystal molecules and fixing alignment of the molecules. The liquid crystal molecules inherently have large birefringence and various alignment forms. Therefore, an optical compensatory sheet obtained from the liquid crystal molecules has a specific optical characteristic that cannot be obtained from the conventional stretched birefringent polymer film.
The optical characteristic of the optical compensatory sheet is designed according to that of the liquid crystal cell, namely, according to display mode of the liquid crystal cell. If the optical compensatory sheet is made with liquid crystal molecules (particularly, discotic liquid crystal molecules), various optical characteristics can be designed according to the display mode of the liquid crystal cell.
Various optical compensatory sheets using discotic liquid crystal molecules have been proposed for liquid crystal cells of various display modes. For example, an optical compensatory sheet for liquid crystal cell of TN mode is described in Japanese Patent Provisional Publication No. 6(1994)-214116, U.S. Pat. Nos. 5,583,679, 5,646,703 and German Patent Publication No. 3,911,620A1. An optical compensatory sheet for liquid crystal cell of IPS or FLC mode is described in Japanese Patent Provisional Publication No. 10(1998)-54982. An optical compensatory sheet for OCB or HAN mode is described in U.S. Pat. No. 5,805,253 and International Patent Application No. WO96/37804. An optical compensatory sheet for STN mode is described in Japanese Patent Provisional Publication No. 9(1997)-26572. An optical compensatory sheet for VA mode is described in Japanese Patent No. 2,866,372.
In preparation of a liquid crystal display, the parts of the display such as the liquid crystal cell, the polarizing plate and the optical compensatory sheet are laminated in order with an adhesive. The optical directions (e.g., polarizing axis, slow axis) of the cell, the polarizing plate and the compensatory sheet must be strictly adjusted according to the displaying mode of the cell. It is, however, impossible to avoid producing some failed displays in which the optical directions of the laminated parts are improperly arranged. The parts in such display are preferably delaminated to reuse them.
The optical compensatory sheet is often destroyed while the sheet is peeled from the liquid crystal cell. The destroyed sheet cannot be reused. Further, the fragments of the sheet remaining on the liquid crystal cell must be removed to reuse the cell.
Accordingly, it has been wanted to improve the mechanical strength of the optical compensatory sheet without affecting its optical characters.
Japanese Patent Provisional Publication No. 8(1996)-27284 discloses a strong optical compensatory sheet. In preparation of the sheet, discotic liquid crystal molecules having polymerizable groups are aligned, and polymerized to improve the mechanical strength of the sheet.
Japanese Patent Provisional Publication No. 9(1997)-152509 discloses another strong optical compensatory sheet. In preparation of the sheet, polymerizable groups are introduced into not only discotic liquid crystal molecules but also a polymer of an orientation layer provided between a transparent support and an optically anisotropic layer. Accordingly, the polymer and the discotic liquid crystal molecules are co-polymerized along an interface between the orientation layer and the optically anisotropic layer.
Japanese Patent Provisional Publication No. 2000-235117 discloses an optical compensatory sheet in which a transparent support and an optically anisotropic layer are combined with a peel strength of 400 g/cm or more. Inorganic fine particles are added into the transparent support, the orientation layer or the optically anisotropic layer to improve the peel strength.